Non-linear transduction results from the introduction of sufficiently intense, audio-modulated ultrasonic signals into an air column. Self-demodulation or down-conversion occurs along the air column resulting in the production of an audible acoustic signal. This process occurs because of the known physical principle that when two sound waves with different frequencies are radiated simultaneously in the same medium, a modulated waveform including the sum and difference of the two frequencies is produced by the non-linear (parametric) interaction of the two sound waves. When the two original sound waves are ultrasonic waves and the difference between them is selected to be an audio frequency, an audible sound can be generated by the parametric interaction.
Parametric audio reproduction systems produce sound through the heterodyning of two ultrasonic signals (signals in the ultrasound frequency range) in a non-linear process that occurs in a medium such as air. The non-linearity of the medium results in acoustic signals produced by the medium that are the sum and difference of the ultrasonic signals. Thus, two ultrasound signals that are separated in frequency can result in a difference tone that is within the 20 Hz to 20,000 Hz range of human hearing.